Medical stent with variable coil and related methods

ABSTRACT

A medical stent includes a first section which includes a first material, defines a lumen, and includes a first coil completing more than one revolution. The first coil revolves about and is coaxial with an axis, expanding and opening as it revolves from the origin of the first coil. A second section of the stent includes a second material, defines a lumen, and includes a second coil completing at least one revolution. A third section defines a lumen and is located between the first and second sections. The third section includes a co-extrusion of the first and second materials. One of the first or second sections is harder than the other section.

TECHNICAL FIELD

[0001] The present invention relates to medical stents and relatedmethods. More specifically, the invention relates to medical stentshaving one section which is softer than a section at the other end ofthe stent.

BACKGROUND INFORMATION

[0002] Fluid sometimes needs to be drained from a body. For example,urine formed in one or both kidneys might need to be drained into thebladder. One way to accomplish such drainage is to use a medical devicethat conveys the fluid (e.g., urine) through a lumen. Such devicesinclude stents and catheters. Existing stents can be uncomfortable forthe patient, especially when they reside in the ureter between thekidney and the bladder, can be difficult for a medical professional toplace in a patient, or can allow urine from the bladder to move into theureter towards the kidney.

SUMMARY OF THE INVENTION

[0003] The present invention provides medical stents for facilitatingdrainage of fluid and methods for placing such stents. For example, suchstents can be placed in a ureter to facilitate drainage of fluid from apatient's kidney to a patient's bladder. Generally, stents according tothe invention have a “softer” end and a “harder” end. The harder endgenerally resides in the patient's kidney while the softer end generallyresides in the patient's bladder. The harder end can transition to thesofter end in a transition section produced by, for example, aco-extrusion process where deposition of a first material is graduallyceased and deposition of a second is gradually increased. In general,the harder end is suited to retain the stent in the patient's kidneyand/or facilitate placement in a patient, while the softer end is suitedto increase patient comfort and/or retain the stent in the patient'sbladder. Additionally, the softer end can inhibit movement of the stentin the bladder, minimize contact between the stent and the bladder, atleast partially occlude the junction between the bladder and ureter inorder to at least partially prevent retrograde urine flow from thebladder into the ureter both around the stent and through the stent,and/or otherwise minimize reflux of urine through the stent towards thekidney. Such stents also are useful in other situations such as biliarydrainage or, generally, where fluid is drained from one body structureto another body structure or out of the body.

[0004] In one embodiment, a medical stent can include a first sectiondefining a lumen and including a first coil completing more than onerevolution, a second section defining a lumen and including a secondcoil completing at least one revolution, and a third section defining alumen and located between the first and second sections. The firstsection can include a first material having a first durometer value, andthe second section can include a second material having a seconddurometer value. The second durometer value can be greater than thefirst durometer value, and at least a portion of the third section caninclude a co-extrusion of the first and second materials. The first coilcan revolve about and be coaxial with an axis. A distance from a firstpoint to the axis, the first point being at the center of a firstcross-section of the first coil and on a line normal to the axis, can beless than a distance from a second point to the axis, the second pointbeing at the center of a second cross-section of the first coil and on aline normal to the axis and the first point being closer to an origin ofthe first coil than the second point. The third section can be adjacentthe origin of the first coil.

[0005] The embodiment described above, or those described below, canhave any of the following features. The axis can generally extend alongthe third section. The second coil can be offset from the axis. Thethird section can include a shaft. The second coil can be generallyperpendicular to the first coil. The first material can be ethylenevinyl acetate. The first material can have a durometer value of about 70to about 90 on a Shore A scale. The second material can have a durometervalue of about 80 to about 95 on a Shore A scale. A cross-section of thelumen in at least one of the first, second, and third sections can becircular. A cross-section of at least one of the first, second, andthird sections can be circular. At least one of the first, second, andthird section can include a radiopaque material. The second coil canhave an outer diameter of at least about 2.0 cm. The first coil can besized and/or shaped such that at least a portion of the first coilresides at the junction of a bladder and a ureter in a patient. Thefirst coil can be a spiral.

[0006] In another aspect of the invention, a medical stent can include afirst section defining a lumen and including a substantially planarfirst coil completing more than one revolution, a second sectiondefining a lumen and comprising a second coil completing at least onerevolution, and a third section defining a lumen and located between thefirst and second sections. The first section can include a firstmaterial having a first durometer value, and the second section caninclude a second material having a second durometer value. The seconddurometer value can be greater than the first durometer value. At leasta portion of the third section can include a co-extrusion of the firstand second materials. The second coil can be generally perpendicular tothe first coil.

[0007] In another aspect of the invention, a method for placing amedical stent includes inserting a medial stent, including any of thestents described above or below with any of the features described aboveor below, into a ureter. At least a portion of the first coil can resideat the junction of a bladder and a ureter in a patient. At least aportion of the first coil can at least partially occlude the junction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] In the drawings, like reference characters generally refer to thesame parts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating principles of the invention.

[0009]FIG. 1 is a schematic rendering of a stent according to theinvention.

[0010]FIG. 2A is a schematic enlarged side view of one end of the stentof FIG. 1.

[0011]FIG. 2B is a schematic end-on view of the stent of FIG. 1.

[0012]FIG. 3 is a schematic cross section of the stent of FIG. 1.

[0013]FIG. 4 is a table showing examples of measurements of portions ofthe stent of FIG. 1.

[0014]FIG. 5 is a schematic rendering of an alternative embodiment ofone coil of a stent according to the invention.

[0015]FIG. 6 is a schematic end-on view of the coil of FIG. 5.

[0016]FIG. 7 is a schematic rendering of an alternate embodiment of astent according to the invention having a similar coil to that in FIG. 1at one end and a different coil from that in FIG. 1 at the opposite end.

[0017]FIG. 8 is a schematic end-on view of the stent of FIG. 7.

[0018]FIG. 9 is an image of a cross section of the embodiment of FIG. 1taken along section line 9-9.

[0019]FIG. 10 is an image of a cross section of the embodiment of FIG. 1taken along section line 10-10.

[0020]FIG. 11 is an image of a cross section of the embodiment of FIG. 1taken along section line 11-11.

[0021]FIG. 12 is an image of a cross section of the embodiment of FIG. 1taken along section line 12-12.

[0022]FIG. 13 is an image of a cross section of the embodiment of FIG. 1taken along section line 13-13.

[0023]FIG. 14 is a schematic rendering of an alternative embodiment of astent according to the invention.

[0024]FIG. 15 is a schematic top view of the embodiment of FIG. 14.

[0025]FIG. 16 is a schematic end-on view of the embodiment of FIG. 14.

[0026]FIG. 17 is a schematic enlarged end-on view of the embodiment ofFIG. 14.

[0027]FIG. 18 is a schematic view of a proximal section of theembodiment of FIG. 14.

[0028]FIG. 19 is a schematic rendering of one system used to manufacturestents according to the invention.

[0029]FIG. 20 is a table containing inner and outer diameter sizes forcertain embodiments of the invention.

[0030]FIG. 21 is a schematic rendering of the stent of FIG. 1 in akidney, ureter, and bladder.

DESCRIPTION

[0031] The present invention provides medical stents for facilitatingdrainage of fluid and methods for placing such stents. For example, suchstents are placed in a ureter to facilitate drainage of fluid from apatient's kidney to a patient's bladder. Generally, stents according tothe invention have a “softer” end and a “harder” end. The harder endgenerally resides in the patient's kidney while the softer end generallyresides in the patient's bladder. The harder end can transition to thesofter end in a transition section produced by, for example, aco-extrusion process where deposition of a first material is graduallyceased and deposition of a second is gradually increased. As usedherein, the terms “hard” and “soft,” and various grammatical formsthereof, are general terms meant generally to refer to a difference inproperties, including, but not limited to, (1) a difference in thedurometer value of all or some of the material(s) used to construct astent (for example, a higher durometer value of one material used inconstructing a section of a stent, even if one or more other materialsare also used to construct that same section of stent, can mean “hard”and a lower durometer value of one material used in constructing anothersection of a stent, even if one or more other materials are also used toconstruct that same section of stent, can mean “soft”), (2) a differencein the retention strengths of the coils on either end of a stent (forexample, a higher retention strength can mean “hard” and a lowerretention strength can mean “soft”), (3) a difference in stiffness (forexample, a more stiff material/section of stent can be “hard” and a lessstiff material/section of stent can be “soft”), or other differencesbetween material(s) used to construct a stent or between sections of astent that those skilled in the art would consider “hard” and/or “soft.”

[0032] Ureteral stents can be made from a higher durometer material tofacilitate placement and retention in the body. However, these firmerstents may contribute to some patient discomfort issues. Ureteral stentsalso can be made from a lower durometer material in an effort to enhancepatient comfort. However, these softer stents may be difficult to placeand may migrate once placed in the patient's body.

[0033] Stents according to the invention are harder end at one end andsofter at the other end. This construction is desirable because, ingeneral, the harder end is suited for placing the stent in the patient'skidney and/or to retain the stent in the patient's kidney, while thesofter end is suited to increase patient comfort and/or, to a degree,retain the stent in the patient's bladder. Moreover, stents according tothe invention can have a coil at the end of the stent to reside in thekidney that is of a size and/or shape that enhances retention of thestent in the kidney. Also, stents according to the invention can have acoil at the end of the stent to reside in the bladder that inhibitsmotion of the stent within the bladder, that enhances patient comfort byreducing, for example, contact between the stent and the neck of thebladder and/or the floor of the bladder, that at least partiallyoccludes the junction between the bladder and ureter to at leastpartially prevent urine from entering the ureter from the bladder eitherthrough or around the stent, and/or that otherwise minimizes reflux ofurine through the stent towards the kidney. Accordingly, stentsaccording to the invention are designed to incorporate multipledesirable features into a single stent and can include any combinationof these features.

[0034] Referring to FIGS. 1, 2A, and 2B, a schematic representation ofone embodiment of a stent 10 according to the invention is shown.Generally, the stent 10 has three sections 20, 22, 24. A first section24 is located at the proximal end (as used herein, proximal refers tothe end of a stent closest a medical professional when placing a stentin a patient) of the stent 10. A second section 20 is located at thedistal end (as used herein, distal refers to the end of a stent furthestfrom a medical professional when placing a stent in a patient) of thestent 10. A third section 22 is located between the first 24 and secondsections 20 and is generally in the form of a shaft. The location of thesections 20, 22, 24 as shown in FIG. 1 is approximate, emphasis insteadbeing placed on illustrating the principles of the invention.

[0035] The first section 24 has a first coil 14 that makes more than onerevolution. The first coil 14 revolves about an axis 23 and is coaxialwith the axis 23. The axis 23 is shown extending generally along thethird section 22 of the stent 10. Although stents according to theinvention typically are flexible, the stents can be placed in a positionin which the shaft of the stent is generally linear to form an axis. Inthis embodiment, the first coil 14 makes more than two revolutions aboutthe axis 23. However, alternate embodiments can revolve to a greater orlesser extent about the axis 23 (for example, less than approximatelytwo revolutions or more than approximately two revolutions). The firstcoil 14 begins at an origin 25. Typically, the origin of stentsaccording to the invention is the location on the stent where thegenerally straight shaft connects to the coil. However, the origin canbe slightly away from this location in certain embodiments. The firstsection 24 and third section 22 meet at the origin 25 in thisembodiment. As the first coil 14 revolves about the axis 23, it opensoutwardly. Thus, a point 17 that is located at the center of across-section of the first coil 14 closer to the origin 25 and along aline 17 a normal to the axis 23 is closer to the axis 23 than is asecond point 19 that is located at the center of a second cross-sectionof the first coil 14 further from the origin 25 and along a line 19 anormal to the axis 23 (best seen in FIG. 2A). The following measurementsprovide one, non-limiting example of the possible size of the first coil14. One revolution of the first coil 14 has a width C of about 0.1 cm,and two revolutions have a width D of about 0.2 cm. One of the turns ofthe coil 14 is measured to be at an angle E of about 75 degrees from theaxis 23. A height F of one part of the coil 14 is about 0.75 cm, and ata location one revolution from measurement F, a height G of the coil 14is about 1.5 cm.

[0036] The second section 20 has a second coil 12 which also makes morethan one revolution and also is offset from the axis 23 of the firstcoil 14 and the general axis of the stent 10. The second coil 12 has atapered tip. Typically, a second coil is larger than coils in some otherstents. For example, a coil can have a diameter of greater than about1.5 cm, preferably greater than about 1.9 cm (including a diameter ofabout 2.0 cm), and more preferably greater than about 2.4 cm (includinga diameter of about 2.5 cm). This size can enhance retention of a stentin a patient's kidney.

[0037] Holes 16 (only some of the holes are labeled) in the outersurface of the stent 10 are located along the length of stent 10. Theseholes 16 allow the outside environment to communicate with a lumeninside the stent 10. The lumen 50 and the stent's outer diameter 52 canbe shaped in cross-section as a circle (best seen in FIG. 3) or anyother appropriate shape such as an oval or other oblong shape. The holes16 can be placed in many configurations, one of which is shown inFIG. 1. In this configuration, the holes 16 are present in the firstcoil 14 in about its first revolution and not in about its secondrevolution. These holes 16 are approximately evenly spaced apart in thefirst coil 14 and are located along the length of the shaft at intervalsof about 1.5 cm with one rotated 90 degrees from the next. In 4.8 Frenchstents, about two to about four holes 16 are in the second coil 12 andin 6, 7, or 8 French stents, about three to about five holes 16 are onthe second coil 12. These holes can be evenly spaced. A suture may beattached to the first section 24 for placing the stent 10 in a desiredposition as well as removing the stent 10. FIG. 4 provides non-limitingexamples of sizes of various stents according to the invention. Forexample, a 4.8 French stent can have a length along portion A of thestent 10 of about 24, 26, or 28 mm and a length along portion B (thearea in which the tip of the stent 10 tapers) of the stent 10 of about 4mm and can have holes of about 0.26 inches.

[0038] The third section 22 is formed from a coextrusion of thematerial(s) from which the first section 24 is made and the material(s)from which the second section 20 is made. As shown in FIG. 1, atransition section 15 (i.e., where the material(s) making up one portionof the stent transition to the material(s) making up another portion ofthe stent) in the third section 22 is closer to the first coil 14 thanto the second coil 12. However, in alternative embodiments, thetransition section can be located anywhere along the length of thestent. The transition section typically is located between the coils oneither end of the stent and is about 2 cm long to about 10 cm long.However, the transition section can be any length. The first section 24includes a first material having a first durometer. The second section20 includes a second material having a second durometer, which isgreater than the first durometer value. Accordingly, the first sectionis “softer” than the second section. The transition section 15 includesboth the first and second materials, and the first and second materialsare separate, distinct, and associated in an unsymmetrical, irregularconfiguration. In operation, the first coil 14 typically resides in thepatient's bladder, and the second coil 12 typically resides in thepatient's kidney (FIG. 21).

[0039] An alternative embodiment of a first coil 114 to be placed in apatient's bladder is shown in FIGS. 5 and 6. The first coil 114 in thisembodiment is in a generally spiral or funnel shape that expands as itrevolves about an axis 123 from the origin 125 of the first coil 114.Again, a point 117 that is located at the center of a cross-section ofthe first coil 114 closer to the origin 125 and along a line normal tothe axis 123 is closer to the axis 123 than is a second point 119 thatis located at the center of a second cross-section of the first coil 114further from the origin 125 and along a line normal to the axis 123. Thefirst coil 114 in this embodiment makes more than three revolutionsabout the axis 123. The following measurements provide one, non-limitingexample of the possible size of the coil 114. One revolution of thefirst coil 114 has a width H of about 0.33 cm, and three revolutionshave a width I of about 0.99 cm. The turns of the coil 114 spreadoutward at an angle J of about 3 degrees relative to the axis 123. Aheight K of one part of the coil 114 is about 0.5 cm, and another heightL of the coil 114 is about 1.5 cm.

[0040] In a further alternative embodiment of a first coil 314 to beplaced in a patient's bladder, a stent 310 is shown in FIGS. 7 and 8that is substantially similar to that shown in FIG. 1 except for thefirst coil 314. The first coil 314 has a generally spiral or funnelshape that expands as it revolves about an axis 323 from the origin 325of the first coil 314. As above, a point 317 that is located at thecenter of a cross-section of the first coil 314 closer to the origin 325and along a line normal to the axis 323 is closer to the axis 323 thanis a second point 319 that is located at the center of a secondcross-section of the first coil 314 further from the origin 325 andalong a line normal to the axis 323. The first coil 314 makes more thantwo revolutions about the axis 323, and the second coil 312 is generallyperpendicular to the first coil 314. The following measurements provideone, non-limiting example of the possible size of the coil. One of theturns of the first coil 314 is measured to be an angle P of about 37degrees from the axis 323. One revolution of the first coil 314 has awidth Q of about 0.5 cm, and two revolutions have a width R of about 1cm.

[0041] The stent 10 of FIGS. 1, 2A, and 2B is a single piece and issized to fit within a ureter. For example, two types of ethylene vinylacetate (“EVA”) can be extruded to form the stent. In a continuousprocess, the first section 24 is formed from one type of EVA; the thirdsection 22, then, is formed by gradually ceasing the deposition of thefirst type of EVA and gradually increasing the deposition of a secondtype of EVA (creating the transition section 15 in the third section22); and the other end of the stent, the second section 20, is formedfrom the second type of EVA after the first type of EVA has ceased beingextruded. Each type of EVA has a different durometer value, with thefirst type of EVA having a durometer value that is less than thedurometer value of the second type of EVA. The two materials in thethird section 22 are separate, are distinct, and are associated witheach other in an irregular configuration. Additionally, other materialsmay be mixed with the first and/or second types of the EVA prior toextrusion. For example, radiopaque materials, such as bismuthsubcarbonate, and/or colorants can be added. The addition can occur atthe site of manufacture or a supplier can supply the EVA alreadycompounded with the radiopaque material alone or with the colorant aloneor with both the radiopaque material and the colorant. Even if thesematerials are mixed, the fact that one EVA type has a durometer valueless than the second EVA type can mean that the section of the stentformed from the first type of EVA is “softer” than the section of thestent formed from the second type of EVA.

[0042] After extrusion, the curled portions are formed. For example, theextrusion can be placed on a mandrel, shaped in a particular form, andthe extrusion can be formed into a desired shape by heating theextrusion while on the mandrel. Alternatively, the extrusion can be laidinto a plate having a groove cut into it in the shape of the desiredfinal product. The plate is heated from below (for example, with a heatlamp) to form the extrusion into a shape according to the configurationof the groove. Both coils can be formed at the same time using twoadjacent plates, each with a groove for the coil at either end of thestent. The plates are heated at different temperatures, to the extentnecessary, for example, if the two ends of the stent are made fromdifferent material(s), and can be heated for the same length of time.Additionally, after extrusion, holes can be bored into the stent byplacing a nylon core inside the stent to prevent the stent fromcollapsing and drilling through the stent, for example, with a hollowsharpened bit.

[0043] FIGS. 9-13 show a series of cross-sectional views taken along thelength of the stent 10. The approximate position of these cross-sectionsare shown in FIG. 1. It should be understood that the position of thesecross-sections is merely an example. In various embodiments, thetransition section of the medical stent can be relatively short, orrelatively long, depending upon the physical characteristics of thestent that are desired. Additionally, sections taken in variousembodiments may look different than the representations shown in FIGS.9-13, depending upon, for example, the length of the transition section,the materials being extruded, and the method of co-extrusion used tomanufacture the stent. Thus, the cross-sections shown in FIG. 1 andFIGS. 9-13 should be understood to illustrate both one embodiment of theinvention and the general principle whereby the material(s) forming onesection of the stent transition to the material(s) forming the othersection of the stent. These figures show one material mixed with acolorant (for example, EVA and a colorant) (the darker portions of thecross-section) gradually increasing in abundance along the length of atleast part of the stent and a second material not mixed with a colorant(for example, a second type of EVA) (the lighter portions of thecross-section) gradually decreasing in abundance along the length of atleast part of the stent. Some of these views are indicative of the firstand second materials being separate, distinct, and associating in anunsymmetrical, irregular configuration. In certain embodiments, thechange in material composition can occur over any part of the shaft ofthe stent or all of the shaft of the stent. At least one of thematerials can be ethylene vinyl acetate. Additionally, stents accordingto the invention can have several transition zones where materialschange and/or can have more than two materials (or more than twomixtures of materials) that change along the length of the stent. Forexample, the shaft of a stent, or a portion thereof, may or may not bethe same material(s) and/or the same durometer as either of the twocoils. Moreover, each of the shaft and two coils can be formed fromdifferent material(s).

[0044] In certain embodiments, the material(s) that make up the secondsection of the stent (the harder section of the stent) can extend atleast half way down the shaft of the stent, and can extend even further,such that the transition section is closer to the first coil (the coilin the softer section of the stent) than to the second coil (the coil inthe harder section of the stent). Such a configuration enhances theplacement characteristics of a stent because the preponderance of hardmaterial(s) makes the stent stiffer and easier for a medical professionto place. In many embodiments, the transition of material(s) does notoccur in one of the coils such that each coil is formed from a singlematerial (or a single mixture of materials). However, the transition canoccur anywhere along the length of the stent. Also in some embodiments,the inner diameter of the stent is maximized but not so much as toadversely impact the stent's ability to be pushed over a guidewire.

[0045] In an alternative embodiment, and referring to FIGS. 14, 15, 16,17, and 18, a stent 210 is shown having a first section 224 located atthe proximal end of the stent 210. A second section 220 is located atthe distal end of the stent 210. A third section 222 is located betweenthe first 224 and second sections 220 and is generally in the form of ashaft. The location of the sections 220, 222, 224 as shown in FIGS. 14and 18 is approximate, emphasis instead being placed on illustrating theprinciples of the invention.

[0046] The first section 224 has a first coil 214 that makes more thanone revolution. The first coil 214 revolves about an axis 223 and iscoaxial with the axis 223. The axis 223 is shown extending generallyalong the third section 222 of the stent 210. Although stents accordingto the invention typically are flexible, the stents can be placed in aposition in which the shaft of the stent is generally linear to form anaxis. In this embodiment, the first coil 214 makes more than tworevolutions about the axis 223. However, alternate embodiments canrevolve to a greater or lesser extent about the axis 223 (for example,less than approximately two revolutions or more than approximately tworevolutions). The first coil 214 is attached to the shaft of the stent210 at a neck 211. The neck is slightly curved and is set back from theaxis 223. As the first coil 214 revolves about the axis 223, it revolvesoutwardly and substantially in a single plane. Accordingly, the firstcoil 214 is substantially planar. The following measurements provideone, non-limiting example of the possible size of the first coil 214 andthe neck 211. The neck 211 has a length M of about 0.82 cm, and thegreatest height N of the coil 214 is about 1.5 cm.

[0047] The second section 220 has a second coil 212 which also makesmore than one revolution and also is offset from the axis 223 of thefirst coil 214 and the general axis of the stent 210. The second coil212 is generally perpendicular to the first coil 214 and has a taperedtip. Typically, a second coil is larger than coils in some other stents.For example, a coil can have a diameter of greater than about 1.5 cm,preferably greater than about 1.9 cm (including a diameter of about 2.0cm), and more preferably greater than about 2.4 cm (including a diameterof about 2.5 cm). In this example the second coil has a diameter ofabout 2.5 cm. This size can enhance retention of a stent in a patient'skidney.

[0048] Holes 216 in the outer surface of the stent 210 allow the outsideenvironment to communicate with a lumen inside the stent 210. The holes216 can be placed in many configurations. Holes 216 are shown in thefirst coil 214 in about its first revolution and are approximatelyevenly spaced apart. Holes also can be located in other parts of thestent 210. The lumen and the stent's outer diameter can be shaped incross-section as a circle or any other appropriate shape such as an ovalor other oblong shape. A suture may be attached to the first section 224for placing the stent 210 in a desired position as well as removing thestent 210.

[0049] The third section 222 is formed from a coextrusion of thematerial(s) from which the first section 224 is made and the material(s)from which the second section 220 is made. A transition section 215(i.e., where the material(s) making up one portion of the stenttransition to the material(s) making up another portion of the stent) inthe third section 222 is closer to the first coil 214 than to the secondcoil 212. However, in alternative embodiments, the transition sectioncan be located anywhere along the length of the stent. The transitionsection typically is located between the coils on either end of thestent and is about 2 cm long to about 10 cm long. However, thetransition section can be any length. The first section 224 includes afirst material having a first durometer. The second section 220 includesa second material having a second durometer, which is greater than thefirst durometer value. Accordingly, the first section is “softer” thanthe second section. The transition section 215 includes both the firstand second materials, and the first and second materials are separate,distinct, and associated in an unsymmetrical, irregular configuration.

[0050] Interrupted layer extrusion techniques, gradient-type coextrusiontechniques, or similar techniques can be used to produce any of thetransition sections described above. Such extrusion techniques can beused instead of using joints or welds to bring together two ends of astent, each end having a different physical property than the other end.Such joints or welds can fail during use of the stent and can bedifficult to manufacture. Continuous material extrusion according to theinvention enhances stent integrity while allowing for desired placementand drainage characteristics. Additionally, continuous extrusionproducts tend not to kink in the transition zone as might a stent with abutt-joint or a weld. In general, any type of thermoplastic polymer canbe extruded such as a silicone, a polyurethane, or a polyolefincopolymer such as EVA. In general, in one embodiment of the invention,two types of EVA (at least one type of EVA can be mixed with aradiopaque material and at least one type of EVA can be mixed with acolorant) are extruded to form the stent. In a continuous process, oneend of the stent is formed from one type of EVA (for example, the firstsection 24 in FIG. 1); an intermediate section (for example, the thirdsection 22 in FIG. 1) containing a transition section (for example, thetransition section 15 in FIG. 1), then, is formed by gradually ceasingthe deposition of the first type of EVA and gradually increasing thedeposition of a second type of EVA; and the other end of the stent isformed from the second type of EVA (for example, the second section 20in FIG. 1) after the first type of EVA has ceased being extruded. Eachtype of EVA has a different durometer value. The mixing of the two typesof EVA in the transition section produces a section in which the twomaterials are separate, are distinct, and are associated with each otherin an irregular configuration. After extrusion, the curled portions areformed.

[0051] In more detail and in one example of an extrusion technique asshown in FIG. 19, a gradient-type technique, a first pelletized type ofEVA is placed in a first dryer 50 and a second pelletized type of EVA isplaced in a second dryer 60. The dryers 50, 60 are hoppers to containthe pellets, and, to the extent necessary, to dry the pellets, and eachdryer 50, 60 feeds the pellets to an extruder 52, 62. The two extruders52, 62 melt the pellets, and each of the melted materials passes througha separate adapter 54, 64 to a separate melt pump 56, 66 (which are alsoreferred to as a gear pumps). Each melt pump 56, 66 has a rotary gearwhich allows the melted materials to pass through the pump 56, 66. Acomputer 58 runs two servo motors 55, 65 that control the melt pumps 56,66. The computer 58 controls the revolutions per minute as a function ofthe distance over which a point in the extruded product travels. Thereis a feedback loop between each melt pump 56, 66 and its relatedextruder 52, 62 such that when the pressure between the extruder 52, 62and the melt pump 56, 66 is too high, the extruder 52, 62 shuts off.Each extruder 52, 62 is a slave to its respective melt pump 56, 66. Thetwo separate lines, each containing a different EVA, come together at across-head 68. The cross-head 68 contains lumens that are separate fromeach other except for a relatively short distance in the cross-head 68.This distance is immediately adjacent a die and a tip where the extrudedproduct exits the cross-head 68. The two materials only come togetherimmediately adjacent to the die and the tip. The die dictates the outerdiameter of the extruded product and the tip dictates the inner diameterof the product. The end of the tip is flush with the end of the die. Airis metered into a port that connects with the tip. Air from the tippushes out the outer and inner diameters of the extruded product. Also,the tip is ported to the atmosphere to avoid the extruded product beingflat. The extruded product (emerging from the cross-head 68 according toarrow 70) is then cooled in a quench tank 72, which is a water bath, tofix the product's shape. Next, the cooled product is dried with an airblower 74 and is measured with a laser micrometer 76. The lasermicrometer 76 measures the outer diameter of the extruded product, andother gauges can be used to measure the inner diameter of the extrudedproduct. The laser micrometer 76 is either monitored by an operator oris connected in a feedback control loop to control the final diameter ofthe extruded product. After passing through the laser micrometer 76, theextruded product is pulled through a “puller/cutter” machine 78. Thisdevice 78 pulls at a particular rate to control the shape of theextruded product, such as tapers on the ends of the extruded product,and cuts the extruded product to the correct length for a stent.Finally, a conveyer 80 separates the acceptable and unacceptable finalproducts. Generally, if the diameter of the extruded product is toolarge according to the laser micrometer, the operator or the feedbackloop will act to speed up the puller/cutter, decrease theextruder(s)/melt pump(s) output(s), and/or decrease the internal airsupport provided through the tip. If the diameter of the extrudedproduct is too small, the operator or the feedback loop will act to slowdown the puller/cutter, increase the extruder(s)/melt pump(s) output(s),and/or increase the internal air support provided through the tip. Whenthe adjustments are made, the measurement of the inside diameter of theextruded product can be taken into account.

[0052] This system has at least three features. First, the entire systemhas no valves, and, specifically, the cross-head 68 has no moving partssuch as valves. Second, extrusion can occur in a non-linear fashion,because the computer 58 and servo motors 55, 65 control the melt pumps56, 66 on the basis of distance traveled. Thus, the melt pumps 56, 66are “ramped up” or “ramped down” as necessary. Accordingly, atheoretically infinite gradient of material can be extruded by varyingthe pumping rates of the melt pumps 56, 66. And third, the process forcombining the two EVA materials does not involve production of wastemelted material as a byproduct of manufacture.

[0053] Through this machinery, in a continuous process, one end of thestent is formed from one type of EVA; an intermediate section containinga transition section, then, is formed by gradually ceasing thedeposition of the first type of EVA and gradually increasing thedeposition of a second type of EVA; and the other end of the stent isformed from the second type of EVA after the first type of EVA hasceased being extruded. Each type of EVA has a different durometer value.A radiopaque material and/or a colorant can be added to either of theEVA materials (the addition can occur at the site of manufacture or asupplier can supply the EVA already compounded with the radiopaquematerial, such as bismuth subcarbonate, alone or with the colorant aloneor with both the radiopaque material and the colorant). The mixing ofthe two types of EVA in the transition section results in a section inwhich the two materials are separate, are distinct, and are associatedwith each other in an irregular configuration. After extrusion, thecurled portions are formed. For example, the extrusion can be placed ona mandrel, shaped in a particular form, and the extrusion can be formedinto a desired shape by heating the extrusion while on the mandrel.Alternatively, the extrusion can be laid into a plate having a groovecut into it in the shape of the desired final product. The plate isheated from below (for example, with a heat lamp) to form the extrusioninto a shape according to the configuration of the groove. Both coilscan be formed at the same time using two adjacent plates, each with agroove for the coil at either end of the stent. The plates are heated atdifferent temperatures, to the extent necessary, for example, if the twoends of the stent are made from different material(s), and can be heatedfor the same length of time. Additionally, after extrusion, holes can bebored into the stent by placing a nylon core inside the stent to preventthe stent from collapsing and drilling through the stent, for example,with a hollow sharpened bit. The stent also can be covered in itsentirety with a lubricant. Useful coatings include those that arehydrophilic.

[0054] Various embodiments of medical stents according to the inventioncan have any of a variety of features. A dual durometer stent thatincorporates a higher durometer value material (for example, firm EVA)for the renal coil and that gradually transitions into a lower durometervalue material (for example, soft EVA) for the bladder coil is useful.For example, the “hard” material can be EVA having a durometer value ofabout 80 to about 95 on a Shore A scale, preferably about 87 to about 95on a Shore A scale, and more preferably about 90 on a Shore A scale, andthe “soft” material can be another type of EVA having a durometer valueof about 70 to about 90 on a Shore A scale, preferably about 78 to about90 on a Shore A scale, and more preferably about 86 on a Shore A scale.These values are examples of a more general principle, namely, having astent with a harder end and a softer end. Other materials or EVA havinga durometer value different than that described above can be useful. Insome embodiments, the materials forming the stent, such as the two typesof EVA, are mixed with other materials. For example, as described above,each type of EVA can be mixed with a radiopaque material, such asbismuth subcarbonate, or a colorant. The radiopaque material allows amedical professional to place the stent under the guidance of an x-raydevice and fluoroscope or other similar device where the radiopaquematerial appears on a view screen because it blocks or reflects x-rayenergy. The colorant also can be used as a visual cue to a medicalprofessional about the location of the stent in the patient.

[0055] Another way to describe the two ends of the stent are by the coilretention strength of each coil of the stent. For example, suchretention strengths can be used as a measure of the ability to resistmigration within a patient, or, more broadly, as a measure of how “hard”or how “soft” the ends of the stent are. One way to determine retentionstrength is found in American Society for Testing and Materials (ASTM)Designation F 1828-97: Standard Specification for Ureteral Stents,approved Nov. 10, 1997, and published May, 1998, the disclosure of whichis incorporated herein by reference. This specification coverssingle-use ureteral stents with retaining means at both ends, duringshort term use for drainage of urine from the kidney to the bladder.These stents typically have diameters of 3.7 French to 14.0 French,lengths of 8 cm to 30 cm, and are made of silicone, polyurethane, andother polymers. They are provided non-sterile for sterilization andsterile for single-use. It is noted that this ASTM standard excludeslong-term, indwelling usage (over thirty days), use of ureteral stentsfor non-ureteral applications, and non-sterile stents. Nevertheless,even if stents according to the invention meet any of these exclusions,or do not otherwise fall under the scope of this ASTM standard, to theextent those skilled in the art understand it to be reasonable to usethe coil retention strength test method described in this document, thetest method can be used.

[0056] The retention strength test method (section 6.2 of the ASTMdocument) involves using a funnel block submerged in a water bath atapproximately 37 degrees Celsius. The funnel block is a block of TEFLONor DERLIN defining a funnel. The funnel is two inches at its widestdiameter and, in cross section, has walls that form an approximately 60degree angle. The funnel narrows to a bore slightly larger than thespecimen to be tested, and this bore is about 0.675 inches long. Theremust be clearance between the outside diameter of the test specimen andthe inside diameter of the hole in the funnel block through which thespecimen is pulled. For example, for stents of 3.7 to 8.5 French, afunnel bore should be 0.125 inches (3.16 mm) in diameter; for stents of10.0 French, a funnel bore should be 0.159 inches (4.04 mm) in diameter;and for stent of 14.0 French, a funnel bore should be 0.210 inches (5.33mm) in diameter. The test specimen is removed from its sterilepackaging, and the retention means (for example, a coil at the end ofthe stent) of the specimen is straightened with an appropriateguidewire. The test specimen is soaked for at least thirty days and iscut to allow a straight portion of the stent to be inserted upwardsthrough the funnel fixture into the grip of a tensile test machinewithout loading the retention mechanism of the stent to be tested. Priorto inserting the test specimen, the test specimen is submerged in thewater bath for at least one minute to allow it to reach thermalequilibrium. If the material is significantly effected by moisture, thetest specimen should be allowed to equilibrate for a minimum of 24hours. The straight portion of the stent then is inserted through thebottom of the funnel and into the grip. If testing 30 days after openingthe package, the retention means is not straightened prior to testing.Then, the specimen is pulled up through the funnel at 20 inches/minute.The maximum force required to pull the stent completely through thefunnel is recorded.

[0057] In certain embodiments, the bladder coil (for example, butwithout limitation, the first coil 14 in FIG. 1) retention strength isless than or equal to the renal coil retention strength (for example,but without limitation, the second coil 12 in FIG. 1). In otherembodiments, the kidney coil retention strength is less than or equal tothe bladder coil retention strength. Typically, retention strengths ofthe two coils are chosen such that the retention strength of the coilplaced in the kidney is greater than the retention strength of the coilplaced in the bladder. A retention strength of at least 10 gram-force ormore is desirable in many embodiments.

[0058] Some embodiments of stents according to the invention can have anouter diameter from about four to about nine French with lengths of fromabout ten to about thirty centimeters as measured between the coils.FIG. 20 shows an example of some suitable French sizes along with thesize of the inner and outer diameters. Unless otherwise noted, thedimensions in FIG. 20 are in inches. The notation “O.D.” refers to outerdiameter and the notation “I.D.” refers to inner diameter. In certainembodiments, stents with standard outer diameter sizes can have innerdiameters (i.e., the diameter of a lumen) that are larger than standardinner diameters normally present in those standard outer diameters. Thisconfiguration facilitates passage of the stent over the guidewire andincreases the drainage allowed by the stent. For example, a four Frenchstent can have an inner diameter equivalent to that found in a 4.8French stent to increase drainage and to facilitate a 0.35 inch and/or a0.38 inch guidewire, and/or a five French stent can have an innerdiameter equivalent to a six French stent to facilitate a 0.35 inchand/or a 0.38 inch guidewire and increase drainage. The stent can havegraduation marks and stent size imprinted on stent.

[0059] The operation of stents according to the invention are describedin FIG. 21. It should be understood that although one embodiment ofstents according to the invention is described, other embodiments canoperate in a similar fashion. In operation of one embodiment of a stent10, the distal end of the stent 10 is inserted through the bladder 104and ureter 102 into the kidney 100. For example, a medical professionalinserts a guidewire (not shown) through the bladder 104, ureter 102 andkidney 100 of a patient. The stent 10 is placed over the guidewire,thereby uncurling the coils 12, 14 to the straightened position. Thestent 10 slides along the guidewire, and the guidewire is sufficientlystiff to hold the coils 12, 14 in a straight configuration while theguidewire is in the lumen of the stent 10. A pusher (optionally with aradiopaque band) that slides over the guidewire, behind the stent 10,abuts the end of the stent and is used to push the stent 10 over theguidewire. The radiopaque band, if used, allows a medical professionalto view the pusher on a fluoroscope, particularly where it abuts thestent, using x-rays. Additionally, if the stent 10 is radiopaque,placement of the stent in the patient can be confirmed by viewing thestent on a fluoroscope. Once at least a portion of the second section 20is positioned within the kidney 100, the guidewire is withdrawn. If apusher is used, the pusher holds the stent in place while the guidewireis removed. The shape-memory material from which second coil 12 isconstructed allows the second section 20 in a straightened position toreturn to its coiled shape in the kidney 100 once the guidewire iswithdrawn. A similar recoiling of the first coil 14 also occurs in thebladder 104 when the guidewire is withdrawn from that area of the stent10. Thus, the “hard” coil 12 is placed in the kidney 100, and the “soft”coil 14 is placed in the bladder 104. Stents can be provided as a kitwith a guidewire and/or a pusher.

[0060] Additionally, the size and/or shape of the first coil 14 allowthe stent 10 to at least partially occlude the junction 106 between thebladder 104 and the ureter 102 (at the orifice of the ureter at the baseof the trigone), for example, because the first coil 14 tends to plugthe ureter. This occlusion may at least partially prevent urine frompassing around or through the stent 10 from the bladder 104 into theureter 102. Additionally, the size and/or shape of the first coil 14 mayat least partially prevent the stent 10 from touching the neck of thebladder 104 and/or the floor of the bladder 104, and, thus, may at leastpartially prevent the stent 10 from irritating or “tickling” the bladder104. Also, the shape and/or size of the first coil 14 can inhibit motionof the stent within the bladder, minimizing contact with the bladderneck and bladder floor. Such minimization can reduce the frequency ofcontractions of the bladder, increasing comfort. Also, having a minimumnumber of holes in the first coil 14 reduces the number of locationsthat pressure, created when a patient urinates, can force urine backinto the stent. Other forms of a first coil are useful to the extentthey can perform any of the functions described above. For example,those embodiments shown in FIGS. 5-6, 7-8, and 14-18 can perform suchfunctions. Additionally, the relatively large size of the second coil 12enhances retention of the stent 10 in the kidney. The same can be saidof other embodiments of the second coil such as the one shown in FIGS. 7and 8 and 14-18.

[0061] The tapered tip on the second coil 14 (the renal coil) canfacilitate inserting the stent through the passages of the patient'sbody. Additionally, a medical professional can use a suture connected tothe stent 10 to reposition the stent (by pulling on it) when insertingthe stent, and the medical professional can use the suture to remove thestent from the patient. For example, the medical professional eitherleaves the suture inside the patient's body or leaves the end of thesuture outside the body. When the stent 10 is to be removed, the medicalprofessional pulls on the suture, removing the stent. However, a suturedoes not have to be used to remove the stent 10.

[0062] When placed in a patient's body, stents according to theinvention may soften slightly, as might many thermoplastic materialswhen exposed to elevated temperatures, for example, but withoutlimitation, by about 30% or less, or about 20% or less, or about 10% orless, or about 5% or less. However, such softening is not substantial.Softening can be measured by methods known in the art. For example, theASTM test method described herein may be adapted to determine if coilssoften by determining if body temperature conditions cause a decrease inretention strength relative to room temperature conditions. However,other methods may be used.

[0063] An alternative method to straighten the coil 12 of the secondsection 20 is to produce relative movement between a straighteningdevice (e.g., a sheath) and second section 20, such that thestraightening device moves distally relative to the second section 20,thereby uncurling the coil 12 to a straightened position. Once at leastsome portion of the second section 20 is positioned within the kidney100, the straightening device is removed. The second section 20 isconstructed from a shape-memory material. Thus, once the straighteningdevice is withdrawn, the coil 12 in the straightened position returns toits coiled shape. A similar re-coiling of the first coil 14 also occurswhen the straightening device is withdrawn from that area of the stent10. Other modes of inserting and/or straightening a device also areuseful.

[0064] Variations, modifications, and other implementations of what isdescribed herein will occur to those of ordinary skill in the artwithout departing from the spirit and the scope of the invention.Accordingly, the invention is to be defined not only by the precedingillustrative description.

[0065] What is claimed is:

1. A medical stent comprising: a first section defining a lumen andcomprising a first coil completing more than one revolution, the firstsection comprising a first material having a first durometer value,wherein the first coil revolves about and is coaxial with an axis andwherein a distance from a first point to the axis, the first point atthe center of a first cross-section of the first coil and on a linenormal to the axis, is less than a distance from a second point to theaxis, the second point at the center of a second cross-section of thefirst coil and on a line normal to the axis, the first point beingcloser to an origin of the first coil than the second point; a secondsection defining a lumen and comprising a second coil completing atleast one revolution, the second section comprising a second materialhaving a second durometer value, wherein the second durometer value isgreater than the first durometer value; and the third section defining alumen and located between the first and second sections and adjacent theorigin of the first coil, at least a portion of the third sectioncomprising a co-extrusion of the first and second materials.
 2. Thestent of claim 1 wherein the axis generally extends along the thirdsection.
 3. The stent of claim 2 wherein the second coil is offset fromthe axis.
 4. The stent of claim 1 wherein the third section comprises ashaft.
 5. The stent of claim 1 wherein the second coil is generallyperpendicular to the first coil.
 6. The stent of claim 1 wherein thefirst material comprises ethylene vinyl acetate.
 7. The stent of claim 1wherein the first material has a durometer value of about 70 to about 90on a Shore A scale.
 8. The stent of claim 1 wherein the second materialhas a durometer value of about 80 to about 95 on a Shore A scale.
 9. Thestent of claim 1 wherein a cross-section of the lumen in at least one ofthe first, second, and third sections is circular.
 10. The stent ofclaim 1 wherein a cross-section of at least one of the first, second,and third sections is circular.
 11. The stent of claim 1 wherein atleast one of the first, second, and third section comprises a radiopaquematerial.
 12. The stent of claim 1 wherein the second coil has an outerdiameter of at least about 2.0 cm.
 13. The stent of claim 1 wherein thefirst coil is sized such that at least a portion of the first coilresides at the junction of a bladder and a ureter in a patient.
 14. Thestent of claim 1 wherein the first coil comprises a spiral.
 15. A methodfor placing a medical stent comprising: inserting a medical stent into aureter, the medical stent comprising: a first section defining a lumenand comprising a first coil completing more than one revolution, thefirst section comprising a first material having a first durometervalue, wherein the first coil revolves about and is coaxial with an axisand wherein a distance from a first point to the axis, the first pointat the center of a first cross-section of the first coil and on a linenormal to the axis, is less than a distance from a second point to theaxis, the second point at the center of a second cross-section of thefirst coil and on a line normal to the axis, the first point beingcloser to an origin of the first coil than the second point; a secondsection defining a lumen and comprising a second coil completing atleast one revolution, the second section comprising a second materialhaving a second durometer value, wherein the second durometer value isgreater than the first durometer value; and the third section defining alumen and located between the first and second sections and adjacent theorigin of the first coil, at least a portion of the third sectioncomprising a co-extrusion of the first and second materials.
 16. Thestent of claim 15 wherein at least a portion of the first coil residesat the junction of a bladder and a ureter in a patient.
 17. The stent ofclaim 16 wherein at least a portion of the first coil at least partiallyoccludes the junction.
 18. A medical stent comprising: a first sectiondefining a lumen and comprising a substantially planar first coilcompleting more than one revolution, the first section comprising afirst material having a first durometer value; a second section defininga lumen and comprising a second coil completing at least one revolution,the second coil being generally perpendicular to the first coil, thesecond section comprising a second material having a second durometervalue, wherein the second durometer value is greater than the firstdurometer value; and a third section defining a lumen and locatedbetween the first and second sections, at least a portion of the thirdsection comprising a co-extrusion of the first and second materials. 19.A method for placing a medical stent comprising: inserting a medicalstent into a ureter, the medical stent comprising: a first sectiondefining a lumen and comprising a substantially planar first coilcompleting more than one revolution, the first section comprising afirst material having a first durometer value; a second section defininga lumen and comprising a second coil completing at least one revolution,the second coil being generally perpendicular to the first coil, thesecond section comprising a second material having a second durometervalue, wherein the second durometer value is greater than the firstdurometer value; and a third section defining a lumen and locatedbetween the first and second sections, at least a portion of the thirdsection comprising a co-extrusion of the first and second materials.